1. Field of the Invention
The present invention pertains to variable oscillators and, in particular, to oscillators operating at relatively high frequencies, such as Colpitts oscillators and the like. Normally, to vary the operating frequency of a crystal oscillator, a variable impedance is connected into the tank circuit. The warp range or operating range, of the oscillator is determined by the amount that this impedance in the tank circuit can be varied. At present, it is common to use a voltage variable capacitor or the like for the variable impedance in the tank circuit so that the frequency of the oscillator can be controlled by a remotely generated voltage. However, the range of voltage variable capacitors is severely limited and, therefore, it is difficult to obtain the desired warp range for the oscillator by using a voltage variable capacitor.
Further, once a relatively wide warp range is obtained in a crystal oscillator, difficulties arise because a crystal has a tendency to be non-linear when utilized over a relatively wide range of frequencies. This non-linearity is caused by the physical capacitance, C.sub.o, of the crystal and can be improved by neutralizing the C.sub.o of the crystal.
A second problem which arises when a crystal oscillator is warped or varied over a wide frequency range is the tendency of the oscillator to oscillate at spurious modes. This tendency to operate at spurious modes, or instability of the oscillator, greatly reduces the usable range of the oscillator. This is especially true for high frequency oscillators.
2. Description of the Prior Art
In the prior art, attempts have been made to improve the linearity of the overall oscillator by neutralizing the C.sub.o of the crystal by placing inductances in parallel therewith. In some instances, such as in U.S. Pat. No. 3,571,754, entitled "Wide Deviation Voltage Controlled Crystal Oscillator", issued Mar. 23, 1971, attempts have been made to incorporate the neutralization coils into a Norton impedance transforming network to increase the deviation range of a crystal oscillator. However, a relatively low frequency type of crystal oscillator is utilized and the difficulties of high frequency operation and instability of the oscillator were not considered or encountered.